Re: [PATCH v7 11/12] firmware: qcom: scm: clarify the comment in qcom_scm_pas_init_image()

[Bjorn Andersson]

On Mon, Feb 05, 2024 at 07:28:09PM +0100, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
> 
> The "memory protection" mechanism mentioned in the comment is the SHM
> Bridge. This is also the reason why we do not convert this call to using
> the TZ memory allocator.
> 

No, this mechanism predates shmbridge.

> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
> Tested-by: Andrew Halaney <ahalaney@xxxxxxxxxx> # sc8280xp-lenovo-thinkpad-x13s
> Tested-by: Deepti Jaggi <quic_djaggi@xxxxxxxxxxx> #sa8775p-ride
> Reviewed-by: Elliot Berman <quic_eberman@xxxxxxxxxxx>
> ---
>  drivers/firmware/qcom/qcom_scm.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/firmware/qcom/qcom_scm.c b/drivers/firmware/qcom/qcom_scm.c
> index 839773270a21..7ba5cff6e4e7 100644
> --- a/drivers/firmware/qcom/qcom_scm.c
> +++ b/drivers/firmware/qcom/qcom_scm.c
> @@ -563,9 +563,13 @@ int qcom_scm_pas_init_image(u32 peripheral, const void *metadata, size_t size,
>  	struct qcom_scm_res res;
>  
>  	/*
> -	 * During the scm call memory protection will be enabled for the meta
> -	 * data blob, so make sure it's physically contiguous, 4K aligned and
> -	 * non-cachable to avoid XPU violations.

What this is saying is that the memory will be made unaccessible to
Linux, so it needs to be contiguous and aligned.

> +	 * During the SCM call the hypervisor will make the buffer containing
> +	 * the program data into an SHM Bridge.

Are you saying that the hypervisor will convert this memory to a
shmbridge, and then pass it to TrustZone?

> This is why we exceptionally
> +	 * must not use the TrustZone memory allocator here as - depending on
> +	 * Kconfig - it may already use the SHM Bridge mechanism internally.
> +	 *

"it may already"? You describe above that we shouldn't pass shmbridge
memory, and the other case never deals with shmbridges. So, I think you
can omit this part.

> +	 * If we pass a buffer that is already part of an SHM Bridge to this
> +	 * call, it will fail.

Could this be because the consumer of this buffer operates in EL2, and
not TZ?

Regards,
Bjorn

>  	 */
>  	mdata_buf = dma_alloc_coherent(__scm->dev, size, &mdata_phys,
>  				       GFP_KERNEL);
> -- 
> 2.40.1
>